6,352 research outputs found
Volatility and dividend risk in perpetual American options
American options are financial instruments that can be exercised at any time
before expiration. In this paper we study the problem of pricing this kind of
derivatives within a framework in which some of the properties --volatility and
dividend policy-- of the underlaying stock can change at a random instant of
time, but in such a way that we can forecast their final values. Under this
assumption we can model actual market conditions because some of the most
relevant facts that may potentially affect a firm will entail sharp predictable
effects. We will analyse the consequences of this potential risk on perpetual
American derivatives, a topic connected with a wide class of recurrent problems
in physics: holders of American options must look for the fair price and the
optimal exercise strategy at once, a typical question of free absorbing
boundaries. We present explicit solutions to the most common contract
specifications and derive analytical expressions concerning the mean and higher
moments of the exercise time.Comment: 21 pages, 5 figures, iopart, submitted for publication; deep
revision, two new appendice
Entanglements in Quiescent and Sheared Polymer Melts
We visualize entanglements in polymer melts using molecular dynamics
simulation. A bead at an entanglement interacts persistently for long times
with the non-bonded beads (those excluding the adjacent ones in the same
chain). The interaction energy of each bead with the non-bonded beads is
averaged over a time interval much longer than microscopic times but
shorter than the onset time of tube constraints . Entanglements
can then be detected as hot spots consisting of several beads with relatively
large values of the time-averaged interaction energy. We next apply a shear
flow with rate much faster than the entangle motion. With increasing strain the
chains take zigzag shapes and a half of the hot spots become bent. The chains
are first stretched as a network but, as the bends approach the chain ends,
disentanglements subsequently occur, leading to stress overshoot observed
experimentally.Comment: 19 pages, 11 figure
Theoretical description of a DNA-linked nanoparticle self-assembly
Nanoparticles tethered with DNA strands are promising building blocks for
bottom-up nanotechnology, and a theoretical understanding is important for
future development. Here we build on approaches developed in polymer physics to
provide theoretical descriptions for the equilibrium clustering and dynamics,
as well as the self-assembly kinetics of DNA-linked nanoparticles. Striking
agreement is observed between the theory and molecular modeling of DNA tethered
nanoparticles.Comment: Accepted for publication in Physical Review Letter
Thermal Fluctuations and Rubber Elasticity
The effects of thermal elastic fluctuations in rubber materials are examined.
It is shown that, due to an interplay with the incompressibility constraint,
these fluctuations qualitatively modify the large-deformation stress-strain
relation, compared to that of classical rubber elasticity. To leading order,
this mechanism provides a simple and generic explanation for the peak structure
of Mooney-Rivlin stress-strain relation, and shows a good agreement with
experiments. It also leads to the prediction of a phonon correlation function
that depends on the external deformation.Comment: 4 RevTeX pages, 1 figure, submitted to PR
Skating on a Film of Air: Drops Impacting on a Surface
Drops impacting on a surface are ubiquitous in our everyday experience. This
impact is understood within a commonly accepted hydrodynamic picture: it is
initiated by a rapid shock and a subsequent ejection of a sheet leading to
beautiful splashing patterns. However, this picture ignores the essential role
of the air that is trapped between the impacting drop and the surface. Here we
describe a new imaging modality that is sensitive to the behavior right at the
surface. We show that a very thin film of air, only a few tens of nanometers
thick, remains trapped between the falling drop and the surface as the drop
spreads. The thin film of air serves to lubricate the drop enabling the fluid
to skate on the air film laterally outward at surprisingly high velocities,
consistent with theoretical predictions. Eventually this thin film of air must
break down as the fluid wets the surface. We suggest that this occurs in a
spinodal-like fashion, and causes a very rapid spreading of a wetting front
outwards; simultaneously the wetting fluid spreads inward much more slowly,
trapping a bubble of air within the drop. Our results show that the dynamics of
impacting drops are much more complex than previously thought and exhibit a
rich array of unexpected phenomena that require rethinking classical paradigms.Comment: 4 pages, 4 figure
The Hydrodynamic Interaction in Polymer Solutions Simulated with Dissipative Particle Dynamics
We analyzed extensively the dynamics of polymer chains in solutions simulated
with dissipative particle dynamics (DPD), with a special focus on the potential
influence of a low Schmidt number of a typical DPD fluid on the simulated
polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid
can lead to underdevelopment of the hydrodynamic interaction in polymer
solutions. Our analyses reveal that equilibrium polymer dynamics in dilute
solution, under a typical DPD simulation conditions, obey the Zimm model very
well. With a further reduction in the Schmidt number, a deviation from the Zimm
model to the Rouse model is observed. This implies that the hydrodynamic
interaction between monomers is reasonably developed under typical conditions
of a DPD simulation. Only when the Schmidt number is further reduced, the
hydrodynamic interaction within the chains becomes underdeveloped. The
screening of the hydrodynamic interaction and the excluded volume interaction
as the polymer volume fraction is increased are well reproduced by the DPD
simulations. The use of soft interaction between polymer beads and a low
Schmidt number do not produce noticeable problems for the simulated dynamics at
high concentrations, except that the entanglement effect which is not captured
in the simulations.Comment: 27 pages, 13 page
Autocorrelation of Random Matrix Polynomials
We calculate the autocorrelation functions (or shifted moments) of the
characteristic polynomials of matrices drawn uniformly with respect to Haar
measure from the groups U(N), O(2N) and USp(2N). In each case the result can be
expressed in three equivalent forms: as a determinant sum (and hence in terms
of symmetric polynomials), as a combinatorial sum, and as a multiple contour
integral. These formulae are analogous to those previously obtained for the
Gaussian ensembles of Random Matrix Theory, but in this case are identities for
any size of matrix, rather than large-matrix asymptotic approximations. They
also mirror exactly autocorrelation formulae conjectured to hold for
L-functions in a companion paper. This then provides further evidence in
support of the connection between Random Matrix Theory and the theory of
L-functions
Theoretical and numerical study of the phase diagram of patchy colloids: ordered and disordered patch arrangements
We report theoretical and numerical evaluations of the phase diagram for a
model of patchy particles. Specifically we study hard-spheres whose surface is
decorated by a small number f of identical sites ("sticky spots'') interacting
via a short-range square-well attraction. We theoretically evaluate, solving
the Wertheim theory, the location of the critical point and the gas-liquid
coexistence line for several values of f and compare them to results of Gibbs
and Grand Canonical Monte Carlo simulations. We study both ordered and
disordered arrangements of the sites on the hard-sphere surface and confirm
that patchiness has a strong effect on the phase diagram: the gas-liquid
coexistence region in the temperature-density plane is significantly reduced as
f decreases. We also theoretically evaluate the locus of specific heat maxima
and the percolation line.Comment: preprint, 32 pages, 6 figures, 3 tables, J. Chem. Phys. in pres
Topological analysis of polymeric melts: Chain length effects and fast-converging estimators for entanglement length
Primitive path analyses of entanglements are performed over a wide range of
chain lengths for both bead spring and atomistic polyethylene polymer melts.
Estimators for the entanglement length N_e which operate on results for a
single chain length N are shown to produce systematic O(1/N) errors. The
mathematical roots of these errors are identified as (a) treating chain ends as
entanglements and (b) neglecting non-Gaussian corrections to chain and
primitive path dimensions. The prefactors for the O(1/N) errors may be large;
in general their magnitude depends both on the polymer model and the method
used to obtain primitive paths. We propose, derive and test new estimators
which eliminate these systematic errors using information obtainable from the
variation of entanglement characteristics with chain length. The new estimators
produce accurate results for N_e from marginally entangled systems. Formulas
based on direct enumeration of entanglements appear to converge faster and are
simpler to apply.Comment: Major revisions. Developed near-ideal estimators which operate on
multiple chain lengths. Now test these on two very different model polymers
Scaling of Entropic Shear Rigidity
The scaling of the shear modulus near the gelation/vulcanization transition
is explored heuristically and analytically. It is found that in a dense melt
the effective chains of the infinite cluster have sizes that scale sub-linearly
with their contour length. Consequently, each contributes k_B T to the
rigidity, which leads to a shear modulus exponent d\nu. In contrast, in phantom
elastic networks the scaling is linear in the contour length, yielding an
exponent identical to that of the random resistor network conductivity, as
predicted by de Gennes'. For non-dense systems, the exponent should cross over
to d\nu when the percolation length becomes much larger than the
density-fluctuation length.Comment: 4 pages, 2 eps figure
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